Clutches for motor vehicles are typically released by axial displacement of a clutch release bearing connected with an apply spring. The apply spring biases the pressure plate toward the flywheel. The release bearing is either, depending on the clutch configuration, pushed toward or pulled away from the clutch cover to overcome the clutch apply force attributable to the apply spring. The bearing is able to sustain high axial thrust loads. An inner race of the bearing rotates with the clutch cover, pressure plate and spring which rotate together as a unit. An outer race of the release bearings does not rotate and is connected to a mechanism used to produce the axial displacement of the bearing. A very common mechanism for axially displacing a release bearing is a pivotably mounted release yoke or release fork in engagement with the bearing in combination with a linkage connecting the yoke with a driver operated clutch pedal. When the clutch pedal is depressed, the release yoke is pivoted, axially displacing the release bearing against the force of the force of the apply spring to disengage the clutch. To reengage the clutch, the pedal is released, allowing the apply spring to return the release bearing to the engaged position.
A number of mechanisms have been developed which employ an electric motor for displacing the release bearing. One such mechanism uses an electric motor to pivot the release yoke in place of the clutch pedal and the intervening linkage. While this configuration may be feasible for light duty applications, such as passenger cars, it is difficult to adapt such a system to accommodate the high loads associated with a heavy duty truck clutch. Other clutch systems employ motors concentric with the transmission input shaft. The motor rotatively displaces a lead screw or a nut on a lead screw which rotates as a unit with the rotor of the motor to axially displace the release bearing. The rotating member must complete several rotations of the associated electric motor rotor to achieve the desired axial displacement of the release bearing, making it difficult to precisely control both the amount and the speed of axial displacement. Controlling both of these parameters is important to achieving optimal clutch engagements which are needed to minimize clutch wear. Additionally, the types of motors illustrated are conventional direct current (DC) motors requiring brushes to conduct current to the motor windings. These brushes will eventually wear out, resulting in motor failure.
It is desired to provide a motor vehicle clutch employing a release bearing displacement mechanism which is electrically actuated and is of simple design, highly reliable, and precisely controllable.